Switching means



March 1947- D. c. PRINCE SWITCHING MEANS Filed Sept. 26, 1942 2Sheets-Sheet l Inventor:

e C u .m m r r IJ m W v .s a DW uu b March 4, 1947. D. c. PRlNCESWITCHING MEANS Filed Sept. 26, 1942 2 Sheets-Sheet 2 Inventor: DavidCPrince,

His Attorney 9 if F: M11146 L a l I M0 w m Patented Mar. 4, 19472,418,951 SWITCHING MEANS David 0. Prince, Schenectady, General ElectricCompany,

New York N. Y., aaaignor to a corporation of Application September 26,1942, Serial No. 459,792 8 Claims. (01. 200-88) My invention relates toswitching means and specifically to an arrangement; in which in a singledevice are provided all the switching and protective means required forprotecting high voltage apparatus.

It is an object of my invention to provide an improved switching meansthat is compact, electrically efficient, durable and weatherproof, andwill interrupt both large and small currents satisfactorily without anyexternal evidence of arcing.

Further objects and advantages of my invention will become apparent asthe following description proceeds and the features of novelty whichcharacterize my invention will be pointed out with particularity in theclaims annexed to and forming a part of this specification.

For a better understanding of my invention reference may be had to theaccompanying drawing in which Fig. 1 is a curve diagram whichillustrates an operating characteristic desired of the switching andprotective means of my invention; Fig. 2 is a side elevational view of aswitching means or device embodying my invention shown in the closedcircuit position; Fig. 3 is a view similar to Fig. 2 showing theswitching means in the open circuit position; Fig. 4 is across-sectional view of the switching means of Fig. 2; Fig. 5 is anenlarged cross-sectional view of a portion of Fig. 4 showing theswitching means in the open position; Fig. 6 is a view taken on line 6-6of Fig. 5, Fig. '7 is a view taken on line 1-1 of Fig. 5; Fig. 8 is anenlarged perspective view of the latch mechanism of the switching meansillustrated in the position shown in Fig. 4, Fig. 9 is a perspectiveview of the bracket for supporting the latch mechanism of Fig. 8, andFig. 10 is a perspective view of a portion of a fuse hook which may beused to replace the blown fuse device'used in my invention.

Referring now to Fig. 1 of the drawings, the preferred form of timecurrent characteristic of switching and protective means for mostgeneral purposes is of the nature of the curve AB of Fig. 1. With a timecurrent characteristic of this sort the circuit is cleared only after ashort time delay in the event of small overloads and this time becomesshorter as the loads increase. This characteristic of a protectivedevice is very desirable since it permits such devices to be connectedin series with one another to give selective operation for faults indifferent locations and furthermore permits the coordination of circuitbreakers, fuses, and the like in a large system. For motor operation,particularly, it is necessary to postpone the operation of a protectivedevice such as a circuit breaker or fuse, so as not to interrupt thecircuit due to the starting currents of motors which may be severaltimes normal running currents. However, an ideal form of tripping curveor time current characteristic should have a positive current limitationat some point and an ideal form of tripping curve would be that shown bythe full lines ABD in Fig. 1. This characteristic has a positive currentlimitation beginning at the point B. This ideal condition can beaccomplished by superimposing the operating characteristic of a currentlimiting fuse of the type disclosed in United States Letters Patent2,188,816, assigned to the same assignee as the present application andhaving the time current characteristic CBD in Fig. 1 upon some devicehaving the time current characteristic AB. My invention is particularlyconcerned with a new and improved switching and protective means whichoperates with a time current characteristic represented by the curve ABDin Fig. l, which I accomplish by providing in a single device thefeatures of a high interrupting capacity current limiting fuse, a switchin series with the fuse which necessarily opens when the fuse operates,and means for interrupting overloads of insufllcient magnitude to causefuse operation. In other words, my invention is concerned with providingin a single device all the switching and protective means ordinarilyrequired in the high voltage circuit of a moderate or low capacitytransformer.

Referring now to Figs. 2, 3, and 4 of the drawings, I have illustrated aswitching and protective means generally indicated at l0, which isadapted to be mounted on a suitable insulator ll only partially shown.For supporting switching means ill on insulator ll there is provided anadapter l2 suitably bolted to insulator II. To adapter l2 in turn isbolted a supporting member l3 including a clamping portion whichcooperates with a cooperating clamp member ll firmly to support theswitching and protective means of my invention. Supported by clampmember I4 is a terminal strip I! to which one terminal such as It of anexternal circuit may be connected.

The switching and protective means III includes an outer tubularhousing, or casing i1 preferably formed entirely of or lined with aninsulating material from which gas is evolved when subjected to the heatof an electric arc. Preferably tubular housing or casing I! which ismounted in a vertical position is made of an arcresisting material suchas fiber or the like. As illustrated in the drawings tubular housing orcasing I! is supported between the cooperating clamping portions ofmembers l3 and 4.

Referring specifically to Figs. 4 and 5, the switching and protectivedevice l of my invention includes an inner tube or movable casing oftubular form, generally indicated at l8, which is adapted to movevertically within tubular casing As illustrated in Figs. 4, 5, 6 and 7of the drawings, inner casing 98 is constructed of a gas evolvinginsulating material and comprises an upper tubular member We and a lowertubular member I81), the adjacent ends of which are threadedly engagedwith a conducting member is having a flange 20 which acts as the movablecontact of the switching device it embodying my invention. Thisconducting member it] also includes an inner integral concentriccup-shaped member We supported by the webs itb best shown in Figs. and7. When the switching device it is in the closed or current-carryingposition indicated in Fig. 4 the flange 2b which will hereinafter hereferred to as the movable contact is in engagement with a contactplunger 29 forming a portion of the stationary contact of the switchingand protective means it of my invention. The contact plunger 28 ismounted for reciprocal movement in an extension M01 of clamp member I4near an opening 22 in outer casing I! so that contact plunger 2| maypass through opening 22 to engage movable contact 20. In order to insuregood contact engagement between contact plunger 2i and movable contact20 a suitable biasing means such as spring 23 is provided to biascontact plunger 2| in a direction so as to tend to cause movementthereof into outer tubular casing ll through opening 22. Good electricalconnection between contact plunger 2| and terminal strip i5 is insuredby means of a flexible conductor 26 interconnecting these members. Theelectrical connection between the flange 2d and terminal strip i5 may befurther improved if desired by means of an adjustable screw contact 25threadedly mounted as indicated at 26 in support it. A suitable lock nut21 may also be included. Contact plunger 2i biases flange 28 againstscrew contact 25.

It will be obvious from the description thus far that without additionalmeans gravity will tend to cause inner casing it to move downwardlyrelative to outer tubular casing H to cause contact 20 and plungercontact 28 to be separated as indicated in Fig. 5. In order to holdcontacts 20 and 2| in engagement as shown'in Fig. 4, it is necessary toprovide suitable latching means generally indicated at 36, best shown inFig. 8, which latching means includes a supporting bracket 3H best shownin Fig. 9. This latching means 30 will be described in greater detailhereinafter.

The upper end of outer tubular casing ll is closed by means of asuitable closure member 32 which is removably fastened to casing H bymeans of a threaded flange 53. A suitable rod 34 connected to endclosure member 32 supports a stationary member 35 which extends aconsiderable distance into the upper end of inner casing l8. Rather thandepend upon gravity to cause downward movement of inner casing llrelative to outer casing I8 I provide a compression spring means 36mounted between stationary support 35 and the upper end 38a of bracket Mwhich bracket has the lower end Mb threadedly mounted as by screws 37(see Figs. 5 and 6) to a plurality of the webs 69b of conducting member-I9 which are provided with cooperating threaded openings 38 (see Fig.7.) The ends talc and 36b of bracket 8| are interconnected by flanges8|c and 3| d, best shown in Figs. 6 and 9.

The lower end of inner casing I8 is closed by means of a conductingmember or contact 39 i which is threadedly engaged with tubular memberl8b. Conducting member or contact 39 is electrically connected withconducting member l9 by means of a conductor 40, winding 4|, andconductor 42. The lower end of conductor 42 is clamped to contact 39 bymeans of a screw 43 threadedly engaging a cup-shaped supporting memberand guide 44 which in turn supports a magnetic structure 45 associatedwith winding 4i. Magnetic structure 45 and winding 4| form a solenoidwhich, depending upon the energize.- tion thereof, controls the positionof a movable armature 46 movable in guide 44. Normally armature 46 is inthe lowermost position indicated in Figs. 4 and 5, but upon apredetermined energization of winding 4|, it is moved upwardly, strikingflange 41 which acts both as a stop and a guide. Flange 4'! is suitablysupported from the magnetic structure 45 as clearly shown in Fig. 5.Conductor 40 which interconnects the upper end of winding 4| withconducting member I9 is fastened to cup-shaped portion |9a of conductingmember 9 by means of a flanged member 48 which acts both as a clamp forconductor 40 and also as a guide as will be brought out in greaterdetail hereinafter. A suitable nut 49 best shown in Figs. 5 and 7, isthreadedly engaged with flanged member 48 to insure good electricalconnection between conductor 40 and conducting member 09.

As was mentioned above, in order that the switching and protective meansof my invention may have the time current characteristic over a portionof the range of operation thereof represented by the curve CBD of Fig.1, I provide as an essential part thereof a current limiting fuse,generally indicated at 50, and including terminals or ferrules 5! ateither end thereof. Preferably this current limiting fuse is of theknown type disclosed in United States Letters Patent 2,188,816,mentioned above. In order that fuse 50 may be connected with theswitching and protective means it for ready removal and replacement, thecontact 39 i provided with a threaded opening 52 for engaging a threadedextension 53 which may be suitably welded or otherwise connected to theupper terminal 5| of current limiting fuse 58. To insure good contactbetween this upper terminal 56 of current limiting fuse 5d and contact39, asuitable spring washer 54 may be provided if desired. With thisarrangement it will be obvious that ready replacement of fuse 50 uponblowing or rupture thereof can be accomplished. The lower terminal 5i offuse 50 may be provided with a fitting or contact device 55, bestshownlin Figs. 2 to 4,

which is clamped to lower fuse terminal 5| and which makesline contactwith a cooperating contact member 56 threadedly connected to the lowerend of outer tubular casing I1. Cooperating contact 56 is preferablyprovided with a movable member 51 and spring 58 to give the desiredcontact wipe. Contact member 56 is electrically connected with aterminal strip 59 and a flexible conductor 60. Another terminal. such as5|, of an external circuit may be connected with terminal strip 59 asshown. Fitting or contact device 55 is provided with a looped extension55' which is adapted to cooperate with the fuse hook shown in Fig. 10,to be described in greater detail hereinafter.

well- Under high current conditions the .circuit through switching meansIt is interrupted by currentlimiting fuse 58 within a fraction of acycle, and subsequent release of latching means 88 to permit separationof contacts 20 and 21 will not cause any arcing. On the other hand,under low current conditions which are not of sufilcient magnitude tocause operation of current limiting fuse 58, interruption of the circuitis accomplished by extinguishment of the arc drawn between contacts 28and 2| which is confined within the annular bore between outer casing 11and inner casing 18. The close proximity of this arc to casings l1 andI8 will cause gas to be evolved from the insulating material which willaid in the rapid extinguishment of the arc confined within the annularbore. Furthermore the close confinement of this are will cause veryrapid cooling thereof, thereby aiding in the circuit interruptingprocess. Upon relative movement of inner tube 18 and outer tube 11 tothe position indicated in Figs. 3 and 5, the current limiting fuse 58 ispushed downwardly to assume the position indicated in Fig. 3. Thisextended position of current limiting fuse 58 gives a clear visual.indication that switching means has operated and that resetting thereofand perhaps replacement of current limiting fuse 58 is necessary. Thecurrent limiting fuse 58 will be provided with a suitable indicator asdisclosed in the above mentioned Letters Patent 2,188,816 which willimmediately indicate to the operator whether or not replacement of thefuse is necessary. Current limiting fuse 58 is, of course, readilyaccessible so that the operator may remove it and insert a new fuse unitusing a switch or fuse hook such as is shown in Fig. 10 and will bedescribed hereinafter.

The latching means 80 supported by bracket 8| for maintaining contacts20 and 2| in engagement, is best shown in Figs. 5, 8, and 9. This latchmechanism is supported within inner casing l8 below stationary member35. The latch means 38 comprises a stationary U-shaped member 53 whichis attached by any suitable means to stationary member 35. Bracket 31 isprovided with a cooperating opening 64 in portion Ila thereof andopenings 65 in portions 3Ic and 31d thereof through which U-shapedmember 83 may extend when the switching means 10 is in the closedposition indicated in Fig. 4. Pivotally mounted at 86 on flanges 31c and31d of bracket 8| is a latch member 81 having a recess 81a. which isadapted to cooperate with the closed end of U-shaped member 83 when thecircuit breaker is latched closed as shown in Figs. 4 and 8. Ifcounterclockwise rotation of latch member 51 as viewed in Fig. 8 isprevented its recess 51a will engage U- haped member 63 so as to holdinner tube l8 relative to outer tube 11 in the position indicated inFig. 4 against the pressure of spring means 85. If, on the other hand,rotation of latch member 61 in the counterclockwise direction ispermitted, its U-shaped recess 81a loses engagement with U-shaped rod 63and the inner tube 18 under the force of spring means 88 movesdownwardly relative to outer tube 18 thereby permitting separation ofcontacts 20 and Latch 61 is prevented from rotating in thecounterclockwise direction by a notch 51b in the free end thereof whichis adapted to be engaged by a trigger member 88 pivotally mounted onshaft 8-8 supported by flanges 81c and 81d of bracket II. Trigger member88 is provided with a notch 18 which when rotated to the properposition, permits release of latch 81. Normally a torsion spring 11associated with shaft 69 rotates shaft 88 and trigger 58 to a positionwhich counterclockwise rotation of latch member 81 is caused uponrelease thereof by rotation of trigger member 68 by bent rods or fingers12 mounted on trigger member 68 which engage pins 13 on either side oflatch member 61, clearly shown in Fig. 8. Torsion spring 11 causesrotation of trigger member 68 to a position which prevents disengagementof \notch 61b of latch member 61 and counterclockwise rotation of shaft69 is necessary to release latch member 61which counterclockwiserotation of trigger member 88 must be accomplished against the force oftorsion spring 11.

In order to operate trigger means 68, a suitable crank 14 is provided atone end of shaft 59 which includes a pin 15 extending through a slot 16in flange 310 of bracket 31. This pin 15 in turn engages with a slot 11in a lever arm 18 pivotally mounted at 18 by flanges ale and 31d ofbracket 31. Sleeve spacer 19' holds lever 18 in the position indicatedin Fig. 8. Normally pin 15 of crank 14 is held in the lower position ofnotches 16 and 11 as shown in Figs. 5 and 8 by torsion spring .11. If,however, lever 18 is caused to rotate in a clockwise direction as viewedin Fig. 8 about pivot 19, pin 15 is moved upwardly, producingcounterclockwise rotation of trigger member 68 to disengage notch 81b oflatch member 61 occurs with the consequent separation of contacts 28 and21. It will be obvious that a very slight motion of trigger member 88 isall that is necessary to accomplish the release of latching means 88.

Latching means 30 may be released in either of two different ways.. The;first way which is the manual releasing method is accomplished by meansof a plunger mounted for vertical movement on end closure member 32.Plunger 88 is preferably connected to manually operable lover or eyemember 8| which is pivotally mounted on end closure member 82 as at 82.Eye member 81 is constructed so that it may be readily operated bypulling down thereon by means of a switch stick having a hook' which maybe inserted through the opening in the eye member 81. Rod 80 ispreferably adjustably secured'to eye member or lever 8| so that downwardmovement of rod 88 is caused by downward movement of lever 81. Asuitable stop 83 is provided to limit the downward movement which stopis adjustable as shown. In order to maintain rod 81 in the upperposition except when force is applied by pulling downwardly on lever 81,I provide a suitable spring means 84. Downward movement of rod 88 willcause clockwise rotation of lever 18 about pivot 19 with the resultingrelease of latch member 51 and consequent interruption of the circuit byseparation of contacts 20 and 2 .1 of switching means 10.

The second method of releasing latch 80 is accomplished through upwardmovement of vertically movable armature 48 associated with the solenoidcomprising winding ll and magnetic structure 45. Armature 46 isconnected to a rodshaped member 85 which extends upwardly in innercasing 18 through flange 41 which acts as a guide and flange member 48which also acts as a guide as well as through nut 49 having a centralopening therein. A suitable spring means 88 which engages a nut 81 onrod 85 normally biases armature l6 and rod 85 in a downward direction.

The upper end of rod 85 is provided with an ad- Justable member 88threadedly mounted thereon which engages directly with pin 15 of lever14 to eil'ect a release of latch means 30. Upward motion of armature 46and rod 85 is caused by overload current in winding 4|. This upwardmovement is against the action of spring means 86.

With the above described arrangement, the release of latching means 30which is a necessary condition before relative movement between innercasing is and outer casing l1, can occur with consequent separation ofcontacts 20 and 2| can be accomplished by three different ways. Thefirst condition under which release of latching means 38 might occur isunder a short circuit condition which is of sufllcient magnitudeto-cause blowing or rupture of current limiting fuse 50. The current isinterrupted by current limiting fuse 50 since its operation is muchfaster than the oper ation of the mechanical means associated withswitching means l0. However, prior to the time the current isinterrupted by current limiting fuse 58, suiiicient flux is produced bywinding 4| to cause armature 46 to move upwardly so that plate 88engages with pin 15 to cause upward movement thereof with consequentrelease of latching means 30. A second way in which release of latchingmeans 30 may be accomplished is through the occurrence of sustainedoverload which will not be sufficient to cause operation of currentlimiting fuse 50 but the current flowing in winding 4| will producesufflcient magnetic flux to cause armature 46 to rise and trip latchingmeans 30. The third means by which latching means 30 may be tripped isthrough the manual operation of plunger 80 as was described in detailabove.

In order to insert a new fuse 50 upon rupture or blowing thereof, Iprovide a fuse or switch hook 9|) as shown in Fig. 10 having a bellmouth or recess 9| into which the looped extension 55' of member 55 isadapted to be inserted. Fuse hook 90 also includes arm 93 and a hookmember 92. It will be obvious that the hook 92 can be inserted in theloop or eye of looped extension 55 as well as in the eye 8| to move thefuse unit upwardly or to pull down on eye 8|. Consequently the bellmouth 9| of fuse hook 90 will fit closely around looped extension 55' offitting 55 by means of which the fuse 50 may readily be lifted intoposition and screwed or unscrewed with respect to contact 39. It shouldbe understood that the looped extension 55 provides sufiicient clearancebelow lower fuse terminal 5| so that the fuse indicator of the typedisclosed in the above mentioned Letters Patent 2,188,816 will havesuflicient clearance to operate and furthermore, so that the conditionthereof may readily be observed by the operator or attendant. I

In view of the detailed description included above the operation of theswitching and pro tective means of my invention will be obvious. With te switching cated in Fig. 4 the latching means 35 maintains the contacts20 and 2| in engagement so that current flows through the switchingdevice from conductor 24, contact plunger 2|, contact 20, conductor 40,winding 4|, conductor 42, contact 39, current limiting fuse 50, contact57, contact device 55, flexible conductor 60 to terminal strip 59. If acurrent of suificient magnitude flows through switching means i0,current limiting fuse 50 will cause very high speed interruption thereofso that subsequent separation of contacts 20 and 2| will not impose anyinterrupting duty on the gap formed interruption by the fuse is rapidcompared with any mechanical motion that can occur. In cases whereseparation of contacts 20 and 2| is initiated due to an overload whichis less than that required to operate current limiting fuse 50, the dutyof interrupting such an overload is imposed on the gap formed betweencontacts 20 and 2|. Successful interruption of such relatively lowtiated manually through operation of lever B the device may be used asa. load breaking switch.

It should be understood that my invention is not limited to the specificdetails and arrangement thereof herein illustrated and that changes suchchanges and modifications as fall within the spirit and scope of myinvention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In an electric switching means, a circuit interrupting devicecomprising an outer casing of insulating material, an inner casing ofinsulating material movably mounted within said outer casing, theexternal transverse dimensions of said ings, a pair of relativelymovable contacts respectively associated with said casings and ar rangedto draw an arc in said annular arc space upon relative separation ofsaid contacts, means for urging relative separation of said contacts,latching means for holding said contacts in current carrying engagementagainst the force of mentioned means, means for reloasing said latchingmeans in response to an abnormal current condition so that said innercasin moves downwardly relative to said outer casing, and meanssupported from said inner casing and adapted to extend out of the lowerend of said outer casing upon downward movement of said inner casing togive a visual indication of operation of said switching means.

2. In an electric switching means, a circuit interrupting devicecomprising a pair of relatively movable contacts, means comprising apair of relatively movable concentrically arranged tubular casings ofinsulating material the external transverse dimensions of the innercasing and the internal transverse dimensions of the outer casingseparation of said contacts is confined, urging relative separation ofsaid contacts, latching means for holding said contacts incurrentcarrying engagement against the force of said last mentionedmeans, means for releasing said latching means in response to anabnormal current condition so that the inner one of said concentricallyarranged tubular casings moves downwardly relative to the outer casing,and means supported from said inner casing and adapted to extend out ofthe lower end of said outer casing upon downward movement of said innercasing to provide a visual indication of the operation of said switchingmeans.

between these contacts since 8. In an electric switching means, acircuit int'erruptingr device comprising a Pair of relatively movablecontacts, means comprising a pair of relatively movable concentricallyarranged tubular casings of insulating material the external transversedimensions of the inner casing and the internal transverse dimensions01' the outer casing being proportioned to provide an annular arc spacewithin which the arc drawnupon relative separation of said contacts isconfined, means for urging relative separation of said contacts,latching means for holding said contacts in current carrying engagementagainst the force of said last mentioned means, means for releasing saidlatching means in response to an abnormal current condition so that theinner one oi said concentrically arranged tubular casings movesdownwardly relative to the outer casing, means supported from said innercasing and adapted to extend out of the lower end of said outer casing'upon downward movement of said inner casing toprovide a visualindication of the'operation of said switching means, said last mentionedmeans comprising a fusible element connected in series with saidcontacts to interrupt the circuit under high current conditions.

4. In an electric switching means, a circuit interrupting devicecomprising a pair of relatively movable contacts, means comprising apair of relatively movable concentrically arranged tubular casings ofinsulating material the external transverse dimensions of the innercasing and the internal transverse dimensions of the outer casing beingproportioned to provide an annular arc space within which an arc isdrawn upon relative separation of said contacts, means mounted withinone of said casings for urging relative separation said contacts,latching means for holding said contacts in current carrying engagementagainst the force of said last mentioned means, means for releasing saidlatching means in response to an abnormal current condition so that theinner one of said concentrically arranged tubular casings movesdownwardly relative to the outer one of said pair of casings, meanssupported from said inner casing so as to extend out of the lower end ofsaid outer casing upon downward movement of said inner casing to provideupon relative separation of said contacts, means 4 supported by andmounted within said inner casing for urging relative separation of saidcontacts, latchin means for holding said contacts in current carryingengagement against the force of said'last mentioned means, means forreleasing said latching means in response to an abnormal currentcondition so that the inner one of said concentrically arranged tubularcasings moves downwardly relative to the outer casing, and meanssupported from said inner casing and adapted to extend out of the lowerend of said outer casing upon downward movement of said inner casing toprovide a visual indication of opl0 eration of said switching means,said last mentioned means comprising a fusible element connected inseries with said contacts to interrupt the circuit under high currentconditions.

6. In an electric switching means, a relatively low capacity circuitinterrupting device compris- I ing an outer casing of insulatingmaterial, an inner casing of insulating material movably mounted withinsaid outer casing, the external transverse dimensions or said innercasing and the internal transverse dimensions 01' said outer casingbeing proportioned to provide an annular arc space between the casings,a pair of relatively movable contacts respectively associated with saidcasings and arranged-to draw an arc in said annular arc space uponrelative separation 01' said contacts, means supported by and mountedwithin said inner casing for urging relative separation of saidcontacts, latchingmeans for holding said contacts in current carryingengagement against the force of said last mentioned means, means forreleasing said latching means in response to an abnormal currentcondition so that the inner one of said concentrically arranged tubularcasings moves downwardly relative to the outer casing, and meanssupported from said inner casing and adapted to extend out of the lowerend or said outer casing upon downward movement of said inner casing toprovide a visual indication of operation of said switching means, saidlast mentioned means comprising a fusible element connected in serieswith said contacts to interrupt the circuit under high currentconditions.

7. In an electric switching means, a circuit interrupting devicecomprising an outer casingof insulating material, an inner casing 01'insulating material movably mounted within said outer casing, theexternal transverse dimensions of said inner casing and the internaltransverse dimensions of said outer casing being proportioned to providean annular arc space between the casings, a pair of relatively movablecontacts respectively associated with said casings and arranged to drawan arc in said annular arc space upon relative separation of saidcontacts, means supported by and mounted within said inner casing forurging relative separation of said contacts, latching means for holdingsaid contacts in current carrying engagement against the force of saidlast mentioned means, means for releasing said latching means inresponse to an abnormal current condition so that the inner one of saidconcentrically arranged tubular casings moves downwardly relative to theouter casing, and means supported from said inner casing and adapted toextend out of the lower end of said outer casin upon downward movementof said inner casing to provide a visual indication of operation of saidswitching means, said last mentioned means comprising a readilyreplaceable current limiting fuse connected in series with said contactsto interrupt the circuit under high current conditions.

8. In an electric switching means, a relatively low capacity circuitinterrupting device comprising an outer casing of insulating material,an inner casing of insulating material movably mounted within said outercasing, the external transverse dimensions of said inner casing and theinternal transverse dimensions of said outer casing being proportionedto provide an annular arc space between the casings, a pair oi?relatively movable contacts respectively associated with said casingsand arranged to draw an arc in said annular arc space upon relativeseparation of said contacts, means supported by and mounted within saidinner casing for urging relative separation of said contacts, latchingmeans for holding said contacts in current carrying engagement againstthe force 01' said last mentioned means, means for releasing saidlatching means in response to an abnormal current condition so that theinner one oi said concentrically arranged tubular casings movesdownwardly relative to the outer casing, and means supported from saidinner casing and adapted to extend out or the lower end 0! said outercasing upon downward movement of said inner casing to provide a visualindication of operation 01' said, switching means, said last mentionedmeans comprising a readily replaceable fuse connected in series with'said contacts to interrupt the circuit under hish current conditions.

DAVID C. PRINCE.

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